1. Field of the Invention
The invention relates to an apparatus and method employing one or more miniature one-piece gas cylinders, for producing low concentration gases for testing and/or calibrating gas monitoring instruments.
2. Description of Related Art
Gas detection instruments are widely used to safeguard human lives and property in various industries in, for example, potentially explosive environments such as mines having methane (CH4) pockets, confined spaces where there exist toxic gases such as carbon monoxide (CO) and hydrogen sulfide (H2S), or a deficiency of oxygen, and chemical process plants where emissions may include sulfur oxide (SOx), nitrogen oxide (NOx), chlorine (Cl2), etc. The heart of each instrument is comprised of sensors that convert chemical energy to electrical energy. The most common sensors include metal oxide semiconductor (MOS) sensors, thermal conductivity sensors, catalytic bead sensors, electrochemical sensors, infrared sensors, and photo ionization detectors. Some types of sensors react with the gas and cause a permanent change in gas molecules and are destructive to the gas sample. Other sensors do not cause a permanent change in gas molecules and are nondestructive technologies. Gas reactions and environmental factors can change sensor performance as well; all sensors are subject to changes in gas sensitivities and sometimes in response times as well. Typically a sensor will gradually lose sensitivity due to environmental or aging effects. In some severe weather conditions, a sensor can lose sensitivity within a very short period of time. For example, a galvanic type oxygen sensor can fail suddenly when the capillary hole, through which oxygen diffuses into the sensor cell, is blocked by water or dust. Some electrochemical sensor can fail rapidly due to leakage of electrolyte. Catalytic bead sensors that are used for detecting combustible gases, can lose sensitivity without any indication after exposure to silicone compounds which poison sensors by building up a solid coating over catalyst sites which blocks gas reactions. The use of a defective, low sensitivity instrument is extremely dangerous as it often gives a false, misleading reading, putting human lives and property in great danger.
In order to maintain gas detection instruments in a working condition, regular calibrations are required. A gas of known concentration, often called a calibration gas (cal gas), must be used as a reference gas. During calibration the instrument is first exposed to clean air free of pollutant, and is then exposed to the calibration gas until a steady state reading is established. The span reading is then compared to the nominal value of gas concentration, and if the reading is the same as the gas concentration then the instrument is considered accurate; otherwise the instrument needs to be corrected. Instrument readout is typically corrected through adjusting a potentiometer in the hardware, or applying a correction factor to raw data in the software. Normally calibrations are performed by trained personnel on a monthly basis. In applications where heavy usage or harsh environments are involved, instruments are required to be calibrated on a more frequent basis, e.g. once a week.
Regular calibrations ensure accuracy of the instrument immediately after calibration. Sensors can, however, fail between calibrations, and instrument users are advised to verify their instrument functionality prior to each use. Such a test is generally referred to as a bump test, in which the instrument is exposed briefly to the gas it's intended to detect to cause a response from the sensor. If the instrument reading is prompt and is within a pre-determined percentage window with reference to the test gas concentration, the instrument is considered to be working properly.
At the present time, most instrument calibrations and tests are done with pressurized, premixed gases, which are supplied from steel or aluminum cylinders. Each cylinder has a valve on the top of the cylinder; upon connecting to a pressure or flow regulator the normally closed valve is open, and when the valve within the attached regulator is opened, the gas is released from the cylinder. Any instrument connected to the system is subsequently exposed to the gas.
Premixed calibration gases are simple to use. Because a gas can be certified before filling a cylinder, no gas mixing is involved thereafter and the use of a pre-mixed calibration gas offers accuracy in gas concentration. If the gas is unstable or reactive, however, it will have a short life and poor accuracy especially when the concentration is in the low parts per million (ppm) range. Such gases include ozone (O3), nitrogen dioxide (NO2), chlorine dioxide (ClO2), chlorine, hydrogen sulfide, etc. Moreover, pre-mixed gases are very expensive. A typical disposable cylinder having 58 liters of gas generally costs between $100–350. Such cylinders are also large in size and are inconvenient to transport and carry in the field.
Typically, a single calibration event consumes between 1–4 liters of calibration gas, so the most popular disposable gas cylinders having 34 or 58 liters of gas would last for only about 10–50 calibrations, at a cost of over $2 for each calibration. In order to cut down the cost, calibration gases are often obtained by diluting gas with mass flow controllers (MFCs) in the laboratory. A MFC controls the rate of gas flow with two pressure transducers placed downstream and upstream, respectively. Two large size cylinders, one of which being a diluting gas, are employed. When the gas of interest and the diluting gas are controlled by two MFCs at preset flow rates, the output gas mixture has a concentration determined by the ratio of the two gas flow rates.
In spite of simple operation and calculation, normal MFCs can't be used for diluting gas accurately at a dilution ratio greater than 10:1. Therefore, the concentration of the source gas must not exceed 10 times that of the desired calibration gas. For example, nitrogen dioxide (NO2) sensors are typically calibrated at 10 ppm by volume. The maximum concentration of NO2 as a source gas for dilution should be 100 ppm, or 0.01% by volume. This means a gas has to be diluted to a very low concentration, either by the gas supplier or by the user, prior to diluting with MFCs in the lab to produce a ppm—level concentration suitable for calibration use. Successive dilutions significantly increase error in the final gas concentration, not to mention the inaccurate concentration of the source gas. According to gas manufacturers, the lower the concentration of a gas in a gas mixture, the higher the error in gas concentration. A gas can be prepared with a tolerance well within ±1–2% when its concentration is above 1% by volume, but it's difficult to make it better than ±5% when its concentration is below 1000 ppm. Besides, mass flow controllers are very expensive, gas generators built with mass flow controllers are usually cost prohibitive.
A pure or concentrated gas makes a much larger quantity of calibration gas than an equal amount of low concentration gas. By using a highly concentrated gas, the volume of the source gas under the same pressure, and thus the size of the cylinder storing the gas and the generator employing the cylinder, can be substantially decreased.
Pure gases in small containers have been employed for calibrating and testing laboratory gas analyzers. They are called permeation devices or permeation tubes. A permeation tube is a small, inert tube containing a pure chemical compound in a two-phase equilibrium between its gas phase and its liquid (or solid) phase. At a constant temperature, the tube emits the compound through its permeable wall at a constant rate, which is then mixed with a carrier gas at a controlled flow rate to obtain a known mixture. Permeation tubes must be used with precise temperature control ovens or apparatus, which are bulky and expensive. They can't be turned off once activated. Although a wide range of permeation rates can be made by varying the length and thickness of the tubes, the normal operating rates range from 5 ng/min to 5 μg/min, or in the parts per billion (ppb) range when diluted to 500 ml/min which is a flow rate typically required for calibrating gas detection instruments in the safety market; they are not, therefore, suitable for gas monitoring applications.
Attempts have been made to generate gases at constant rates immediately before use through photo-chemical, electrochemical, thermo-chemical and other chemical processes. For example, U.S. Pat. No. 3,752,748 discloses an ozone generator, which utilizes a photochemical method. Electrochemical gas generators for generating H2S, H2, Cl2 and some other gases are described in U.S. Pat. No. 5,395,501. The generators comprise an electrochemical cell having gas generating and counter electrodes with an intervening body of electrolyte; electric current passing between the electrodes causes the generation of gas at the gas-generating electrode. In this electrochemical type generator, the gas is generated when needed and the rate of its production, and thus also its concentration in a carrier gas, is controlled through the current that passes through the electrolytic cell.
In addition to the chemical gas generating methods, a gas of low concentration can also be generated from its liquid phase if the boiling point of the species is higher than ambient temperature. For example, U.S. Pat. No. 6,234,001 discloses an apparatus for generating calibration gas comprising a contained stream of carrier gas, and a chamber containing at least part of the stream and a volatile reference liquid are held by a wick structure so that as the stream passes through the chamber, the volatile reference liquid evaporates into a gas that blends into the carrier gas to form a calibration gas.
On-site gas generation methods and apparatus as described are available for a very limited number of gases. They suffer from poor accuracy, too. The most used test gases such as carbon monoxide and methane cannot be produced in a reliable, controlled manner through chemical or evaporation processes.